1. Field of the Invention
This invention relates to an electrophotographic printing apparatus and a method for operating the same. More particularly, the present invention is directed to an improved developing means for forming a toner image and an operating method thereof.
2. Description of the Related Art
There have been developed various electrophotographic printers in which a latent electrostatic image is formed by projecting an optical beam onto a photoconductive layer. By depositing toner particles on the photoconductive layer, the resulting latent electrostatic image is developed into a toner image which is thereafter transferred onto recording paper and fixed thereon.
The principles of various known methods will be described with reference to FIGS. 1-4. FIG. 1 is a block diagram of a prior art electrophotographic printing apparatus which utilizes a process referred to as the Carlson method wherein corona dischargers are employed. A photosensitive drum 101 comprising a photoconductive layer 102 (such as a selenium layer) is rotated in the direction indicated by arrow 100, and the surface of the photosensitive drum 101 is uniformly charged (positively in this case) with ions generated by a corona discharging device 103, as shown in FIG. 1. Subsequently, the photoconductive layer 102 is exposed to an optical beam, such as a laser beam, emitted from an optical image source 104. The resulting electrostatic latent image, corresponding to an object pattern to be reproduced, is developed by depositing electrostatically charged toner particles on the photosensitive layer 102, employing a magnetic brush developer 105. The toner image is electrostatically transferred to recording paper 110 which is charged at the opposite polarity of the toner particles, employing another corona discharger 106. The toner is then fixed on the recording paper 110 with an image fixer 107. The charges retained in the photosensitive layer 102 and residual toner particles remaining on the photosensitive layer 102 are neutralized by a corona discharger 108, and the discharged toner particles are wiped away by a fur brush 109. This completes one cycle of the electrophotographic printing process.
Each corona discharger requires a high voltage, such as several thousand volts, and is very sensitive to the atmospheric conditions, such as the level of humidity, dust and other contaminants contained in the air. In addition, ozone gas is generated during the corona discharge, exposing operators to a health hazard. The use of the corona discharge devices creates problems such as a high cost, an unstable printing operation, and a health hazard to the operators. To overcome the disadvantages described above, an electrophotographic device without corona discharging devices has been recently developed.
For example, an electrophotographic printing apparatus is disclosed in Japanese Laid-Open Provisional Application No. 57-119375, issued on July 24, 1982, to Y. Nishigaki. FIG. 2 is a block diagram illustrating the configuration of the Nishigaki apparatus. A photosensitive film 115 (formed of a photosensitive medium) of, for example, a transparent substrate 111, a transparent electrode 112 formed of ITO (Indium-Tin-Oxide), a photoconductive layer 113 formed of a 65 .mu.m layer of cadmium sulfide (CdS), and a white insulator layer 114 formed of titanium oxide (TiO). The four layers are laminated to each other in the recited order to form the layered photosensitive film 115. A magnetic brush developer 116 is placed adjacent to and facing the photosensitive film 115. An optical beam is emitted from an optical image source including an optical source 117 such as a laser, a rotating polygonal mirror 118, and a lens 119. The optical beam is projected onto a portion of the photosensitive film 115 adjacent to the magnetic brush developer 116, from the transparent substrate (111) side of the photosensitive film 115, making the exposed portion of the photoconductive layer 113 conductive. Since a bias voltage is applied between the magnetic brush developer 116 and the transparent electrode 112, the photo-carriers generated in the exposed portion of photoconductive layer 113 are attracted by an electrostatic force toward the white insulator 114 and blocked thereby, forming an electrostatic latent image. Consequently, the electrostatic field between the electrostatic latent image and the magnetic brush developer 116 is fairly strong. On the other hand, the electric field across the unexposed portion of the photosensitive film (which remains non-conductive) is weak since the photoconductive layer 113 has a large thickness in comparison to the white insulator 114. Thus, charged toner particles carried by the magnetic brush developer 116 in contact with the photosensitive film 115 are attracted to the exposed portion of the photosensitive film 115 and are not attracted to the unexposed portion, forming a visual image on the photosensitive film 115. The formed toner image is transferred to a recording medium. The electric charge of the electrostatic latent image and of the residual toner particles left on the photosensitive film 115 are gradually discharged before the next printing process starts, and are collected magnetically by the magnetic brush developer 116.
Although the Nishigaki electrophotographic printing method advantageously does not require a corona discharging device employing a high voltage, a relatively thick photoconductive layer 113 is required to provide satisfactory contrast, because toner image formation is accomplished by utilizing the difference between the adhering forces generated by electric fields, namely Coulomb forces, in exposed and unexposed areas as described above. The fabrication of a thick photoconductive layer having a uniform thickness is difficult and the cost of materials is high. A reduction in the photo-sensitivity of the photoconductive layer 113 and an increase in the required bias voltage applied between the transparent electrode 112 and magnetic brush developer 116 occur as the thickness of the photoconductive layer 113 increases. In addition, when conductive toner particles are employed, ordinary recording paper having relatively low resistivity cannot be used as a recording medium because the charge of the deposited toner particles can be easily discharged. Thus, a specially treated medium, for example, a paper coated with an insulative layer, must be used.
A further improved electrophotographic printing apparatus which overcomes the above-described disadvantages is disclosed in allowed U.S. patent application, Ser. No. 762,431 by Kimura et al., assigned to the assignee of the present invention. FIG. 3 is a block diagram of the Kimura et al. electrophotographic printing apparatus and includes an electrophotographic printing drum 140, a first magnetic brush developer 125, a second magnetic brush developer 131, an optical image source 128, an optical discharger 137, an image transferring means 135, and an image fixer 138. A photosensitive film 124 is formed on the electrophotographic printing drum 140 and includes a transparent substrate 121, a transparent electrode 122, and a photoconductive layer 123, which are laminated to each other in the recited order. The transparent electrode 122 is grounded. The photosensitive film 124 does not have an insulator layer formed thereover for blocking photo-carriers, because the photoconductive layer 123 has electrical trap potentials underneath the top surface. As a result, the thickness of the photosensitive film 124 is reduced. Such a photoconductive material is commercially available as, for example, an organic photoconductive material supplied by the Eastman KODAK Co. under model number SO-102.
The first magnetic brush developer 125 and the second magnetic brush developer 131 are separated from each other by a predetermined distance. Both magnetic brush developers are conventional, each having a rotating magnet roller and a sleeve of non-magnetic material arranged co-axially. The toner particles employed are magnetically conductive particles or magnetically non-conductive particles which are carried by magnetic particle carriers, and are supplied to the magnetic brush developers 125 and 131. The magnetic toner particles or the particle carriers are magnetized by rotating magnetic fields generated by the rotating magnet rollers, and are formed into a series of toner particle "chains" extending in the radial direction of the sleeve of the developer 125, thus forming a so-called magnetic "brush". The magnetic brush also rotates but in the opposite direction to that of the magnet roller. Bias voltages having opposite polarities are supplied from the power sources 126 and 132, respectively, and are applied to the first and the second magnetic brush developers 125 and 131. The optical image source 128 includes a self-focusing lens (a product of Nippon Plate Glass LTD, commercially available under the brand name SELFOC lens) and an LED array, and emits an optical beam.
During the printing process the printing drum 140 is rotated, in a direction indicated by an arrow 120, at a constant speed. During one cycle of rotation, the following printing steps are performed sequentially:
(1) Toner particles (negatively charged, for example) are attracted to the photoconductive layer 123 by an electric field generated by a bias voltage of negative polarity supplied from the power source 126, developing a uniformly distributed toner image 127 (a solid image) on the surface of the photosensitive film 124.
(2) The solid image 127 is moved to an exposing station 130 where an optical beam emitted from the optical image source 128 is projected onto the photosensitive film 124 from the rear side thereof, i.e., from the transparent substrate 121 side of photosensitive film 124. The exposed portions of the photoconductive layer 123 are made conductive by positive photo-carriers generated in the photoconductive layer 123. The positive photo-carriers reach a trapping potential that exist close to the top surface of the photoconductive layer 123, trapped by the trap potential and fixed therein even after the laser beam is turned off, thus, forming an electrostatic latent image 129 in the photoconductive layer 123.
(3) Using the second magnetic brush developer 131, a reverse bias (positive) voltage is applied to the developer to release the toner particles 134 deposited on the unexposed portion of the photosensitive film 124. The released particles 134 are recovered by the second magnetic brush developer 131. The majority of toner particles on the exposed portion of the photoconductive layer 123 adhere to the surface due to an electrostatic force generated by the trapped charges, even though a small portion of the toner particles may be released. Thus a visual toner image 133 is developed on the photosensitive film 124.
(4) The toner image 133 is then rotated to a transferring station where the toner image 133 is transferred to recording paper 136 by a conventional image transferring means 135, and is fixed on the recording paper 136 by a conventional image fixer 138.
(5) The trapped photocarriers forming the electrostatic latent image 129 are discharged by the optical discharger 137. The remaining toner particles 139 on the photosensitive film 124 are collected by the first magnetic brush developer. 125. Thus, the printing drum 140 is recycled to perform a new printing operation.
In the above-described photoconductive layer 123, the mobility of the photo-carriers by optical exposure is rather slow, requiring a substantial time to complete the formation of the electrostatic latent image. The exposing station 130 is necessary to facilitate such an exposure time. On the other hand, with respect to a photoconductive layer made of, for example, cadmium sulfide (CdS), selenium (SE), and photosensitive organic materials, with photo-carriers of a high mobility, there is no need to facilitate a separate exposure time, and the exposure and the first development can be performed simultaneously. This process is realized in the electrophotographic printing apparatus shown in FIG. 4. The apparatus of FIG. 4 has a further advantage in that the toner particle layer formed at the first magnetic brush developer 125 has a thicker toner image at the exposed portions than the toner particle layer at the unexposed portions, because the electric field at the exposed portions is stronger than that of the unexposed portions. As a result, some image contrast of the toner particle layer appears at this printing stage which is advantageous because it produces a denser toner image.
However, the electrophotographic printing apparatuses of FIG. 3 and FIG. 4 need two magnetic brush developers for solid image developing, producing a complicated structure that is high in cost.
A low voltage electrophotography process and a compact apparatus therefor is disclosed in U.S. Pat. No. 4,545,669, issued on Oct. 8, 1985, to Hays et al. As illustrated in FIG. 3 of the '669 Patent, the apparatus includes a single magnetic brush roller and a flexible belt-like imaging member. The toner "chains" on the magnetic brush roller are moved in the same direction as the belt-like imaging member using a driving roller system. The imaging member is flexible and deflected such that the magnetic brush roller is in contact with the imaging member, securing a contacting length therebetween sufficient to form a `sensitizing nip`, and a `development nip` which is immediately adjacent to the sensitizing nip at a location downstream thereof. On a stationary shell or on a sleeve of the magnetic brush developer, an electrically insulated strip which serves as an electrode for the sensitizing nip, is disposed, at the upper side of the nip. In this configuration, the magnetic brush developer, in cooperation with the bias voltages, performs the functions of the exposure means, the first magnetic brush developer, and the second magnetic brush developer, described above, in the following manner: toner particles supplied to the magnetic brush developer are developed uniformly by an electric field generated by a bias voltage Vs applied to the strip, and simultaneously, an electrostatic latent image is formed on the imaging member at the sensitizer nip by a rear exposure using an optical beam emitted from an electronic imaging source. Thereafter, the toner particles deposited on non-exposed portions of the imaging member are released and scavenged (removed) by an electric field of the opposite gradient to that of the sensitizing nip, and thus a toner image is formed on the surface of the imaging member. The surface speed of the magnetic brush developer is much higher than that of the imaging member, preferably by two to four times. As described previously, a certain amount of time is necessary to create a electrostatic latent image in a photoconductive layer or to develop a toner image. Therefore, a certain distance in which there is contact between the imaging member and the magnetic brush developer is required to allow the necessary time to develop the image. In the apparatus by Hays et al., the distance is provided by utilizing the flexibility of the imaging member employed; that is, the distance in which there is contact between the imaging member and the magnetic brush developer, and thus, the time of contact, is increased by a slight pressing of the magnetic brush developer against the relaxingly tensioned flexible belt-like imaging member. However, if a solid flat imaging member, particularly a photosensitive roller, is employed, there may not be sufficient contact time between the magnetic brush developer and the solid flat imaging member to develop a toner image and scavenge the relevant toner particles. This limitation on the material useable for the imaging member is undesirable and disadvantageous.